linux/block/blk-flush.c
<<
>>
Prefs
   1/*
   2 * Functions to sequence FLUSH and FUA writes.
   3 *
   4 * Copyright (C) 2011           Max Planck Institute for Gravitational Physics
   5 * Copyright (C) 2011           Tejun Heo <tj@kernel.org>
   6 *
   7 * This file is released under the GPLv2.
   8 *
   9 * REQ_{FLUSH|FUA} requests are decomposed to sequences consisted of three
  10 * optional steps - PREFLUSH, DATA and POSTFLUSH - according to the request
  11 * properties and hardware capability.
  12 *
  13 * If a request doesn't have data, only REQ_FLUSH makes sense, which
  14 * indicates a simple flush request.  If there is data, REQ_FLUSH indicates
  15 * that the device cache should be flushed before the data is executed, and
  16 * REQ_FUA means that the data must be on non-volatile media on request
  17 * completion.
  18 *
  19 * If the device doesn't have writeback cache, FLUSH and FUA don't make any
  20 * difference.  The requests are either completed immediately if there's no
  21 * data or executed as normal requests otherwise.
  22 *
  23 * If the device has writeback cache and supports FUA, REQ_FLUSH is
  24 * translated to PREFLUSH but REQ_FUA is passed down directly with DATA.
  25 *
  26 * If the device has writeback cache and doesn't support FUA, REQ_FLUSH is
  27 * translated to PREFLUSH and REQ_FUA to POSTFLUSH.
  28 *
  29 * The actual execution of flush is double buffered.  Whenever a request
  30 * needs to execute PRE or POSTFLUSH, it queues at
  31 * q->flush_queue[q->flush_pending_idx].  Once certain criteria are met, a
  32 * flush is issued and the pending_idx is toggled.  When the flush
  33 * completes, all the requests which were pending are proceeded to the next
  34 * step.  This allows arbitrary merging of different types of FLUSH/FUA
  35 * requests.
  36 *
  37 * Currently, the following conditions are used to determine when to issue
  38 * flush.
  39 *
  40 * C1. At any given time, only one flush shall be in progress.  This makes
  41 *     double buffering sufficient.
  42 *
  43 * C2. Flush is deferred if any request is executing DATA of its sequence.
  44 *     This avoids issuing separate POSTFLUSHes for requests which shared
  45 *     PREFLUSH.
  46 *
  47 * C3. The second condition is ignored if there is a request which has
  48 *     waited longer than FLUSH_PENDING_TIMEOUT.  This is to avoid
  49 *     starvation in the unlikely case where there are continuous stream of
  50 *     FUA (without FLUSH) requests.
  51 *
  52 * For devices which support FUA, it isn't clear whether C2 (and thus C3)
  53 * is beneficial.
  54 *
  55 * Note that a sequenced FLUSH/FUA request with DATA is completed twice.
  56 * Once while executing DATA and again after the whole sequence is
  57 * complete.  The first completion updates the contained bio but doesn't
  58 * finish it so that the bio submitter is notified only after the whole
  59 * sequence is complete.  This is implemented by testing REQ_FLUSH_SEQ in
  60 * req_bio_endio().
  61 *
  62 * The above peculiarity requires that each FLUSH/FUA request has only one
  63 * bio attached to it, which is guaranteed as they aren't allowed to be
  64 * merged in the usual way.
  65 */
  66
  67#include <linux/kernel.h>
  68#include <linux/module.h>
  69#include <linux/bio.h>
  70#include <linux/blkdev.h>
  71#include <linux/gfp.h>
  72
  73#include "blk.h"
  74
  75/* FLUSH/FUA sequences */
  76enum {
  77        REQ_FSEQ_PREFLUSH       = (1 << 0), /* pre-flushing in progress */
  78        REQ_FSEQ_DATA           = (1 << 1), /* data write in progress */
  79        REQ_FSEQ_POSTFLUSH      = (1 << 2), /* post-flushing in progress */
  80        REQ_FSEQ_DONE           = (1 << 3),
  81
  82        REQ_FSEQ_ACTIONS        = REQ_FSEQ_PREFLUSH | REQ_FSEQ_DATA |
  83                                  REQ_FSEQ_POSTFLUSH,
  84
  85        /*
  86         * If flush has been pending longer than the following timeout,
  87         * it's issued even if flush_data requests are still in flight.
  88         */
  89        FLUSH_PENDING_TIMEOUT   = 5 * HZ,
  90};
  91
  92static bool blk_kick_flush(struct request_queue *q);
  93
  94static unsigned int blk_flush_policy(unsigned int fflags, struct request *rq)
  95{
  96        unsigned int policy = 0;
  97
  98        if (blk_rq_sectors(rq))
  99                policy |= REQ_FSEQ_DATA;
 100
 101        if (fflags & REQ_FLUSH) {
 102                if (rq->cmd_flags & REQ_FLUSH)
 103                        policy |= REQ_FSEQ_PREFLUSH;
 104                if (!(fflags & REQ_FUA) && (rq->cmd_flags & REQ_FUA))
 105                        policy |= REQ_FSEQ_POSTFLUSH;
 106        }
 107        return policy;
 108}
 109
 110static unsigned int blk_flush_cur_seq(struct request *rq)
 111{
 112        return 1 << ffz(rq->flush.seq);
 113}
 114
 115static void blk_flush_restore_request(struct request *rq)
 116{
 117        /*
 118         * After flush data completion, @rq->bio is %NULL but we need to
 119         * complete the bio again.  @rq->biotail is guaranteed to equal the
 120         * original @rq->bio.  Restore it.
 121         */
 122        rq->bio = rq->biotail;
 123
 124        /* make @rq a normal request */
 125        rq->cmd_flags &= ~REQ_FLUSH_SEQ;
 126        rq->end_io = rq->flush.saved_end_io;
 127}
 128
 129/**
 130 * blk_flush_complete_seq - complete flush sequence
 131 * @rq: FLUSH/FUA request being sequenced
 132 * @seq: sequences to complete (mask of %REQ_FSEQ_*, can be zero)
 133 * @error: whether an error occurred
 134 *
 135 * @rq just completed @seq part of its flush sequence, record the
 136 * completion and trigger the next step.
 137 *
 138 * CONTEXT:
 139 * spin_lock_irq(q->queue_lock)
 140 *
 141 * RETURNS:
 142 * %true if requests were added to the dispatch queue, %false otherwise.
 143 */
 144static bool blk_flush_complete_seq(struct request *rq, unsigned int seq,
 145                                   int error)
 146{
 147        struct request_queue *q = rq->q;
 148        struct list_head *pending = &q->flush_queue[q->flush_pending_idx];
 149        bool queued = false;
 150
 151        BUG_ON(rq->flush.seq & seq);
 152        rq->flush.seq |= seq;
 153
 154        if (likely(!error))
 155                seq = blk_flush_cur_seq(rq);
 156        else
 157                seq = REQ_FSEQ_DONE;
 158
 159        switch (seq) {
 160        case REQ_FSEQ_PREFLUSH:
 161        case REQ_FSEQ_POSTFLUSH:
 162                /* queue for flush */
 163                if (list_empty(pending))
 164                        q->flush_pending_since = jiffies;
 165                list_move_tail(&rq->flush.list, pending);
 166                break;
 167
 168        case REQ_FSEQ_DATA:
 169                list_move_tail(&rq->flush.list, &q->flush_data_in_flight);
 170                list_add(&rq->queuelist, &q->queue_head);
 171                queued = true;
 172                break;
 173
 174        case REQ_FSEQ_DONE:
 175                /*
 176                 * @rq was previously adjusted by blk_flush_issue() for
 177                 * flush sequencing and may already have gone through the
 178                 * flush data request completion path.  Restore @rq for
 179                 * normal completion and end it.
 180                 */
 181                BUG_ON(!list_empty(&rq->queuelist));
 182                list_del_init(&rq->flush.list);
 183                blk_flush_restore_request(rq);
 184                __blk_end_request_all(rq, error);
 185                break;
 186
 187        default:
 188                BUG();
 189        }
 190
 191        return blk_kick_flush(q) | queued;
 192}
 193
 194static void flush_end_io(struct request *flush_rq, int error)
 195{
 196        struct request_queue *q = flush_rq->q;
 197        struct list_head *running = &q->flush_queue[q->flush_running_idx];
 198        bool queued = false;
 199        struct request *rq, *n;
 200
 201        BUG_ON(q->flush_pending_idx == q->flush_running_idx);
 202
 203        /* account completion of the flush request */
 204        q->flush_running_idx ^= 1;
 205        elv_completed_request(q, flush_rq);
 206
 207        /* and push the waiting requests to the next stage */
 208        list_for_each_entry_safe(rq, n, running, flush.list) {
 209                unsigned int seq = blk_flush_cur_seq(rq);
 210
 211                BUG_ON(seq != REQ_FSEQ_PREFLUSH && seq != REQ_FSEQ_POSTFLUSH);
 212                queued |= blk_flush_complete_seq(rq, seq, error);
 213        }
 214
 215        /*
 216         * Kick the queue to avoid stall for two cases:
 217         * 1. Moving a request silently to empty queue_head may stall the
 218         * queue.
 219         * 2. When flush request is running in non-queueable queue, the
 220         * queue is hold. Restart the queue after flush request is finished
 221         * to avoid stall.
 222         * This function is called from request completion path and calling
 223         * directly into request_fn may confuse the driver.  Always use
 224         * kblockd.
 225         */
 226        if (queued || q->flush_queue_delayed)
 227                blk_run_queue_async(q);
 228        q->flush_queue_delayed = 0;
 229}
 230
 231/**
 232 * blk_kick_flush - consider issuing flush request
 233 * @q: request_queue being kicked
 234 *
 235 * Flush related states of @q have changed, consider issuing flush request.
 236 * Please read the comment at the top of this file for more info.
 237 *
 238 * CONTEXT:
 239 * spin_lock_irq(q->queue_lock)
 240 *
 241 * RETURNS:
 242 * %true if flush was issued, %false otherwise.
 243 */
 244static bool blk_kick_flush(struct request_queue *q)
 245{
 246        struct list_head *pending = &q->flush_queue[q->flush_pending_idx];
 247        struct request *first_rq =
 248                list_first_entry(pending, struct request, flush.list);
 249
 250        /* C1 described at the top of this file */
 251        if (q->flush_pending_idx != q->flush_running_idx || list_empty(pending))
 252                return false;
 253
 254        /* C2 and C3 */
 255        if (!list_empty(&q->flush_data_in_flight) &&
 256            time_before(jiffies,
 257                        q->flush_pending_since + FLUSH_PENDING_TIMEOUT))
 258                return false;
 259
 260        /*
 261         * Issue flush and toggle pending_idx.  This makes pending_idx
 262         * different from running_idx, which means flush is in flight.
 263         */
 264        blk_rq_init(q, &q->flush_rq);
 265        q->flush_rq.cmd_type = REQ_TYPE_FS;
 266        q->flush_rq.cmd_flags = WRITE_FLUSH | REQ_FLUSH_SEQ;
 267        q->flush_rq.rq_disk = first_rq->rq_disk;
 268        q->flush_rq.end_io = flush_end_io;
 269
 270        q->flush_pending_idx ^= 1;
 271        list_add_tail(&q->flush_rq.queuelist, &q->queue_head);
 272        return true;
 273}
 274
 275static void flush_data_end_io(struct request *rq, int error)
 276{
 277        struct request_queue *q = rq->q;
 278
 279        /*
 280         * After populating an empty queue, kick it to avoid stall.  Read
 281         * the comment in flush_end_io().
 282         */
 283        if (blk_flush_complete_seq(rq, REQ_FSEQ_DATA, error))
 284                blk_run_queue_async(q);
 285}
 286
 287/**
 288 * blk_insert_flush - insert a new FLUSH/FUA request
 289 * @rq: request to insert
 290 *
 291 * To be called from __elv_add_request() for %ELEVATOR_INSERT_FLUSH insertions.
 292 * @rq is being submitted.  Analyze what needs to be done and put it on the
 293 * right queue.
 294 *
 295 * CONTEXT:
 296 * spin_lock_irq(q->queue_lock)
 297 */
 298void blk_insert_flush(struct request *rq)
 299{
 300        struct request_queue *q = rq->q;
 301        unsigned int fflags = q->flush_flags;   /* may change, cache */
 302        unsigned int policy = blk_flush_policy(fflags, rq);
 303
 304        /*
 305         * @policy now records what operations need to be done.  Adjust
 306         * REQ_FLUSH and FUA for the driver.
 307         */
 308        rq->cmd_flags &= ~REQ_FLUSH;
 309        if (!(fflags & REQ_FUA))
 310                rq->cmd_flags &= ~REQ_FUA;
 311
 312        /*
 313         * An empty flush handed down from a stacking driver may
 314         * translate into nothing if the underlying device does not
 315         * advertise a write-back cache.  In this case, simply
 316         * complete the request.
 317         */
 318        if (!policy) {
 319                __blk_end_bidi_request(rq, 0, 0, 0);
 320                return;
 321        }
 322
 323        BUG_ON(rq->bio != rq->biotail); /*assumes zero or single bio rq */
 324
 325        /*
 326         * If there's data but flush is not necessary, the request can be
 327         * processed directly without going through flush machinery.  Queue
 328         * for normal execution.
 329         */
 330        if ((policy & REQ_FSEQ_DATA) &&
 331            !(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) {
 332                list_add_tail(&rq->queuelist, &q->queue_head);
 333                return;
 334        }
 335
 336        /*
 337         * @rq should go through flush machinery.  Mark it part of flush
 338         * sequence and submit for further processing.
 339         */
 340        memset(&rq->flush, 0, sizeof(rq->flush));
 341        INIT_LIST_HEAD(&rq->flush.list);
 342        rq->cmd_flags |= REQ_FLUSH_SEQ;
 343        rq->flush.saved_end_io = rq->end_io; /* Usually NULL */
 344        rq->end_io = flush_data_end_io;
 345
 346        blk_flush_complete_seq(rq, REQ_FSEQ_ACTIONS & ~policy, 0);
 347}
 348
 349/**
 350 * blk_abort_flushes - @q is being aborted, abort flush requests
 351 * @q: request_queue being aborted
 352 *
 353 * To be called from elv_abort_queue().  @q is being aborted.  Prepare all
 354 * FLUSH/FUA requests for abortion.
 355 *
 356 * CONTEXT:
 357 * spin_lock_irq(q->queue_lock)
 358 */
 359void blk_abort_flushes(struct request_queue *q)
 360{
 361        struct request *rq, *n;
 362        int i;
 363
 364        /*
 365         * Requests in flight for data are already owned by the dispatch
 366         * queue or the device driver.  Just restore for normal completion.
 367         */
 368        list_for_each_entry_safe(rq, n, &q->flush_data_in_flight, flush.list) {
 369                list_del_init(&rq->flush.list);
 370                blk_flush_restore_request(rq);
 371        }
 372
 373        /*
 374         * We need to give away requests on flush queues.  Restore for
 375         * normal completion and put them on the dispatch queue.
 376         */
 377        for (i = 0; i < ARRAY_SIZE(q->flush_queue); i++) {
 378                list_for_each_entry_safe(rq, n, &q->flush_queue[i],
 379                                         flush.list) {
 380                        list_del_init(&rq->flush.list);
 381                        blk_flush_restore_request(rq);
 382                        list_add_tail(&rq->queuelist, &q->queue_head);
 383                }
 384        }
 385}
 386
 387static void bio_end_flush(struct bio *bio, int err)
 388{
 389        if (err)
 390                clear_bit(BIO_UPTODATE, &bio->bi_flags);
 391        if (bio->bi_private)
 392                complete(bio->bi_private);
 393        bio_put(bio);
 394}
 395
 396/**
 397 * blkdev_issue_flush - queue a flush
 398 * @bdev:       blockdev to issue flush for
 399 * @gfp_mask:   memory allocation flags (for bio_alloc)
 400 * @error_sector:       error sector
 401 *
 402 * Description:
 403 *    Issue a flush for the block device in question. Caller can supply
 404 *    room for storing the error offset in case of a flush error, if they
 405 *    wish to. If WAIT flag is not passed then caller may check only what
 406 *    request was pushed in some internal queue for later handling.
 407 */
 408int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask,
 409                sector_t *error_sector)
 410{
 411        DECLARE_COMPLETION_ONSTACK(wait);
 412        struct request_queue *q;
 413        struct bio *bio;
 414        int ret = 0;
 415
 416        if (bdev->bd_disk == NULL)
 417                return -ENXIO;
 418
 419        q = bdev_get_queue(bdev);
 420        if (!q)
 421                return -ENXIO;
 422
 423        /*
 424         * some block devices may not have their queue correctly set up here
 425         * (e.g. loop device without a backing file) and so issuing a flush
 426         * here will panic. Ensure there is a request function before issuing
 427         * the flush.
 428         */
 429        if (!q->make_request_fn)
 430                return -ENXIO;
 431
 432        bio = bio_alloc(gfp_mask, 0);
 433        bio->bi_end_io = bio_end_flush;
 434        bio->bi_bdev = bdev;
 435        bio->bi_private = &wait;
 436
 437        bio_get(bio);
 438        submit_bio(WRITE_FLUSH, bio);
 439        wait_for_completion_io(&wait);
 440
 441        /*
 442         * The driver must store the error location in ->bi_sector, if
 443         * it supports it. For non-stacked drivers, this should be
 444         * copied from blk_rq_pos(rq).
 445         */
 446        if (error_sector)
 447                *error_sector = bio->bi_sector;
 448
 449        if (!bio_flagged(bio, BIO_UPTODATE))
 450                ret = -EIO;
 451
 452        bio_put(bio);
 453        return ret;
 454}
 455EXPORT_SYMBOL(blkdev_issue_flush);
 456
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.